The present invention relates to LED drivers and, more particularly, to LED matrix driver ghost image prevention apparatus and method.
Light Emitting Diodes or LEDs are often used in visual applications that require time-multiplexing of numerous LEDs in the display. Time-multiplexing is a scheme that involves connecting the cathodes of multiple LEDs to each OUT pin of the LED driver. A time-multiplexed circuit is advantageous because it uses fewer LED drivers for a given amount of LEDs, which results in lower cost and smaller size. One major drawback to time-multiplexing is a side-effect called ghosting. The ghosting phenomenon is caused by stray board capacitance which can force time-multiplexed LEDs to flash when they should be off.
For the purpose of understanding how ghost-image is produced, an exemplary conventional time multiplex LED driver is presented as have been describe in MAXIM application note 411, titled “Eliminating Ghost-currents in Color-LED Display System using the MAX6972-MAX6975 LED Drivers”. Referring to FIG. 1, the multiplexing transistors (Q1 and Q2) are alternately turned on by the LED Driver IC while the constant current-sinking drive pins (OUT1-OUT3) alternate control settings between the two phases. During Phase 1 (FIG. 2), MUX1 pin is low, Q1 is turned on, and node A is pulled to the LED supply, VLED, thereby connecting LED2, LED4, and LED6 anodes to the LED supply. Likewise, during Phase 2, MUX2 is low and Q2 is turned on, connecting LED1, LED3, and LED5 anodes to the LED supply. The MUX1 and MUX2 outputs turn on the PNP transistors by sinking base current through the resistors via their open-drain drivers. When MUX1 and MUX2 are off, the open-drain outputs are essentially open-circuit, allowing the base-emitter resistors to turn off the PNP transistors. Between each MUX1 and MUX2 phase, both Q1 and Q2 are off, which is shown as tEMUX in FIG. 2. Faint ghost images from parasitic currents occur during the transition from MUX1 to MUX2 and vice versa. The effects are most pronounced when the LEDs on the multiplexed circuits are different colors (light wavelengths) and, hence, have significantly different voltage drops for a given current flow. Further referring to FIG. 1, assuming all odd numbered LED are green and even numbered LED are red, and assuming that
The voltage drops of LEDs are:
                VRED=2V        VGREEN=3.1VThe supply voltage:        +VLED=5VAt phase 1, with Q1 turned on, the anode of the red LEDs will be connected to the supply voltage, this will in turned charge the parasitic capacitor Cp1 at node A to approximately 5V. With Outputs OUT1-OUT3 active and assuming the voltage drop of the PNP transistor to be negligible, all LED cathodes will be pulled to a voltage approximately equal to:5V−VRED=3V  (eq. 1)When phase 1 ends, the 3 output drivers will be off and MUX1 will be inactive, disconnecting the anode of the LEDs from the supply voltage. Since there are no discharge paths for the parasitic capacitor the voltage at node A will remain close to the supply voltage. When phase 2 begins, MUX 2 will be low, Q2 turn on, the anode of green LED is connected to 5V, and Outputs OUT1-OUT3 are activated. The voltage at the cathodes of the all LEDs will then be approximately equal to:5V−VGREEN=1.9V  (eq. 2)With all cathode voltage approximately equal to 1.8V, the anode of the red LED will need to discharge to1.8V+VRED=3.8V  (eq. 3)From 5V voltage at node A at the beginning of phase 2 node A will discharge to 3.8V through the red LED. This discharging produces a faint illumination or ghost image on one or more red LEDs.        
The ghost image can be eliminated by providing a discharge path for the parasitic capacitor Cp1-Cp2 and providing time for the discharge to occur. This is accomplished by adding R1 and R2, as shown in FIG. 3. However this method does not give solution to the stray capacitances Cp3-Cp5 present on the OUT pins and the method presented also lowers the efficiency due to the current which are consumed by the additional resistor. Furthermore this technique is limited to only LED time multiplex circuit arrangement as discussed above. For LED time multiplexed circuit where every LED output drives both anodes and cathodes of LEDs, as shown in FIG. 4, this method may not suitable. In this LED driver architecture the driver outputs Z1-Z3 function as a switch to connect the LEDs to the power supply or constant current supply. The main objective of the current invention is to provide the most effective prevention to LED ghost image without sacrificing efficiency.